A vehicle performing idling stop (idling reduction) can considerably improve a fuel efficiency by stopping an idling of an engine. In the vehicle performing regenerative power generation, a battery is charged through driving an alternator by kinetic energy of the vehicle at deceleration. In the vehicle performing idling stop, it is necessary to supply power from the battery to the starter motor at restarting the engine, and the battery supplying power to the starter motor is configured to be charged by the alternator. As the battery supplying power to the starter motor, considering the battery characteristics, a lead-acid battery having a nominal voltage 12 V is generally used. However, as the lead-acid battery has a relatively large charging resistance, it is not efficiently charged with regenerative generation power.
In addition, in the vehicle performing idling stop for improving a fuel efficiency, in a state where an ignition switch as a main switch of the vehicle is held in the ON state, the engine is stopped when the vehicle is stopped due to waiting for the traffic light, and the engine is restarted and the vehicle moves at the time of signal changing. In the vehicle, each time the engine is restarted, power is supplied from the lead-acid battery to the starter motor. As a consuming current of the starter motor is remarkably large, the voltage of the lead-acid battery temporarily remarkably drops at restarting the engine, for example, at less than 9 V, namely a state of the instantaneous voltage drop occurs. When the instantaneous voltage drop in the lead-acid battery occurs, a control unit for a vehicle or other electric equipment which operate by using the power source of the lead-acid battery cannot operate in the normal state temporarily, and there is a demerit that the normal operation cannot be ensured.
In order to resolve this demerit, the power supply device where the lead-acid battery is connected in parallel to a sub-battery is developed. (refer to patent literatures 1 and 2)